Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (a.k.a., syn gas or synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is performed in the presence of a carbonylation catalyst and resulting in the formation of a compound, for example, an aldehyde, which has one more carbon atom in its molecular structure than the starting olefinic feedstock. By way of example, higher alcohols may be produced in the so-called "oxo" process by hydroformylation of commercial C.sub.6 -C.sub.12 olefin fractions to an aldehyde-containing oxonation product, which on hydrogenation yields respective C.sub.7 -C.sub.13 saturated alcohols. The crude product of the hydroformylation reaction will contain catalyst, aldehydes, alcohols, unreacted feed, syn gas and by-products.
Before further processing of the crude product is possible, it is necessary to remove the catalyst therefrom. One conventional method of removing cobalt values from such a crude product is to treat the product with an alkali or acid wash technique. See U.S. Pat. No. 3,725,534 (Reisch), which issued on Apr. 3, 1973. However, this approach uses expensive raw materials and incurs problems associated with finally removing essentially all traces of cobalt from the water wash streams before being discharged.
Another conventional method involves the oxidation of the cobalt catalytic species followed by extraction as a cobalt salt in aqueous solution. See U.S. Pat. No. 2,744,921 (Mertzweiller et al.), which issued on May 8, 1956.
U.S. Pat. No. 4,625,067 (Hanin), which issued on Nov. 25, 1986, discloses still another method which involves the contacting of the crude product with a stream of stripping gas to entrain volatile cobalt, characterized in that the contacting is performed in the presence of water and aqueous acid to dissolve those cobalt values not entrained in the gas under the conditions of temperature and pressure employed for the contacting, and the aqueous phase is subsequently separated from the organic hydroformylation reaction product.
Although the stripping method disclosed in the Hanin patent overcomes the disposal and chemical additive costs of the caustic/acidification method of Reisch, it has the disadvantage that low concentrations of cobalt are absorbed into the olefinic feedstock per unit volume requiring the use of a large absorber unit and substantially all of the olefinic feedstock.
Therefore, conventional absorber systems are acceptable for applications where substantially all of the olefinic feedstock is fed through the absorber such that all of the available cobalt is returned to the oxo reactor together with the feedstock. However, instances where it is not desirable to pass all of the olefinic feedstock through the absorber section, conventional absorber systems have been found to be incapable of returning all of the available cobalt to the oxo reactor. This is because conventional absorbers operating under typical absorber conditions (i.e., 30.degree. C. and 0.8 atm CO partial pressure) are only capable of absorbing approximately 0.3 weight % of cobalt (i.e., 3,000 ppm by weight of cobalt metal).
In some instances it is highly desirable to preheat a portion of the olefinic feedstock prior to its introduction into the oxo reactor. The preheated feedstock allows the hydroformylation reaction to proceed immediately without the necessary heat-up period. This can substantially decrease the overall time that it takes for the oxo reaction to proceed. However, if the cobalt-containing olefinic feedstock from the absorber is preheated, then the volatile cobalt compound contained therein will tend to plate out as highly undesirable cobalt metal.
The present inventor has developed an absorber system that is capable of absorbing substantially more cobalt per unit volume of olefinic feedstock than convention absorber systems, i.e., a cobalt concentration of up to approximately 3.5 weight %. This cobalt concentration level is unattainable via conventional absorber systems.
The present inventor has developed a novel method and system for substantially increasing the cobalt concentration per unit volume of olefinic feedstock. This is accomplished by recycling a portion of the discharged liquid phase back to the absorber via a liquid hold-up reactor under predetermined operating conditions such that the concentration of non-volatile cobalt compound species in the liquid phase is increased due to the decomposition of the volatile cobalt compound.
The present invention attains such cobalt concentration levels by the incorporation of a means for passing the liquid phase discharged from the absorber unit through a liquid hold-up reactor and either returning the cobalt enhanced solution to the absorber or sending it on to the oxo reactor. The volume, temperature and pressure of the liquid hold-up reactor are adjusted so as to increase the rate at which volatile HCo(CO).sub.4 decomposes to a non-volatile species such acylcobalt carbonyl and/or Co.sub.2 (CO).sub.8. The present inventor has also discovered that the use of linear olefins as the feedstock also substantially increases the rate at which volatile cobalt compounds decompose to non-volatile cobalt compound species.
The present invention also provides many additional advantages which shall become apparent as described below.